The present invention relates to machines for drawing metallic rods or tubes, and more particularly to improvements in combined drawing and straightening machines for rod-shaped or tubular metallic stock. Still more particularly, the invention relates to improvements in draw benches wherein the stock moves lengthwise and is treated by one or more reducing dies.
It is already known to provide a draw bench with means for effecting at least some straightening of tubular or metallic workpieces simultaneously with a reduction of the diameter. For example, German Pat. No. 977,295 discloses a drawing machine wherein the reducing die is located downstream of a ring which is movable radially to a number of different positions but remains at a standstill once it assumes a selected position. The ring cooperates with the reducing die to produce a certain straightening effect. However, each adjustment of the ring takes up a substantial amount of time, mainly because the final adjustment must be arrived at on the basis of trail and error. Thus, it is necessary to draw a rod or tube through the reducing die and to observe the straightening action or the absence of straightening action of the ring. If the operator determines that the workpiece is being flexed in a given direction, the ring is shifted radially of the reducing die in the opposite direction and the trial is repeated in order to determine the extent of straightening action of the ring in its new position. As a rule, the ring must be adjusted at least twice so that the total time which elapses for a satisfactory adjustment is much too long for an economical drawing and straightening operation. Furthermore, even an optimum adjustment of the ring cannot eliminate at least some flexing of the reduced workpiece so that the latter must be subjected to a costly and time-consuming secondary straightening action. Still further, the just described ring is incapable of furnishing a satisfactory straightening action if the characteristics of the workpiece are not identical in each and every increment thereof; for example, changes in strength of successive increments of a rod-shaped or tubular workpiece, as well as changes in wall thickness of at tubular workpiece are likely to greatly affect the staightening action. Such changes cannot be observed as the workpiece passes through the draw bench so that the final product is bent in spite of placing of the straightening ring in an optimum position with respect to the reducing die. Therefore, many plants employing draw benches prefer to dispense with the straightening ring, especially for the drawing of relatively small numbers of workpieces, because the time spent for proper adjustment of the ring plus the time spent for secondary straightening of workpieces would render the operation uneconomical. Such smaller lots are normally treated in a draw bench without any straightening means and are thereupon introduced into a conventional straightening machine.
The publication "Steel in the USSR" (July 1971, pages 558-559) discloses a combined draw bench and straightening machine wherein the straightening means comprises two concentric axially movable guide sleeves one of which is located upstream and the other of which is located downstream of an orbiting reducing die. A drawback of such proposal is that the reducing must orbit about the common axis of the two guide sleeves and that it must be installed in one or more thrust bearings which take up stresses arising when a workpiece is being pulled through the reducing die. This means that the draw bench must employ very large, bulky and highly expensive thrust bearings. The initial and maintenance cost is further increased due to the fact that the draw bench must be equipped with precision finished adjusting means for changing the eccentricity of the reducing die. Such draw benches cannot be used for the treatment of tubular stock whose internal diameter is determined by a suitable mandrel because the mandrel in the interior of the tubular workpiece would have to move sideways in response to orbital movement of the reducing die.